Soda lime

Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO₂ retention and carbon dioxide poisoning.

It is made by treating slaked lime with concentrated sodium hydroxide solution.

The main components of soda lime are

During the administration of general anaesthesia, the gases expired by a patient, which contain carbon dioxide, are passed through an anaesthetic machine breathing circuit filled with soda lime granules. Medical-grade soda lime includes an indicating dye that changes color when the soda lime reaches its carbon dioxide absorbing capacity.

To ensure that a soda lime canister (CO₂ absorber) is functioning properly, it should not be used if the indicating dye is activated. Standard anaesthesia machines typically contain up to 2 kg of soda lime granules.

Lithium hydroxide (LiOH) is the alkali hydroxide with the lowest molecular weight (24 g/mol; Li: 7 g/atom gram) and is therefore used as CO₂ absorbent in space flights since the Apollo programme to spare weight at launch. During Apollo 13 flight, the crew sheltered in the lunar module started suffering from high CO₂ levels and had to adapt spare absorbent cartridges from the Apollo capsule to the LEM system.

Recent generation of CO₂ absorbents have been developed to reduce the risk of formation of toxic by-products as a result of the interaction between the absorbent and inhaled anesthetics. Some absorbents made from lithium hydroxide (LiOH) are also available for this purpose.

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